watchdog: hpwdt (8/12): implement WDIOC_GETTIMELEFT

Let applications check the amount of time left before the watchdog will fire.

Signed-off-by: dann frazier <dannf@hp.com>
Acked-by: Thomas Mingarelli <Thomas.Mingarelli@hp.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>